This invention relates to novel solder compositions or solder creams, and is particularly concerned with the provision of solder compositions especially adapted for soldering components in electronic circuitry, which substantially eliminates the undesirable formation of solder balls.
A solder cream is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical joint or interface commonly described as a solder joint. The solder cream generally consists of metallic balls, e.g. of 45-70 micron diameter, of various alloys such as tin-lead, tin-lead-silver, tin-lead-gold, etc., contained in a vehicle including a flux comprised of rosin such as wood rosin, or derivatives thereof. In addition, a suspension medium is present in the vehicle and is combined with the flux to provide a paste substance of uniform texture, consistency, and improved surface wetting. In addition, metallic corrosion and metallic settling is retarded, by incorporation of additional components in the vehicle.
In electronic circuitry, solder creams or compositions are employed to secure miniature electronic components to a circuit on a substrate such as ceramic or glass, or the like. Circuits similar to those used on printed circuit boards are produced on the substrates. In certain instances, the circuit is located on one side of the substrate, with pads being provided in certain areas, other parts of the circuit being covered with an insulating material.
The solder cream having a paste-like consistancy is applied, for example by means of a silk screen, to the pads on the circuit board. Thereafter, the electronic components are carefully positioned with their peripheral contacts on the solder cream-coated pads. When all of the components are thus in place, the board, with such components temporarily positioned and retained thereon by the solder cream, can be placed in a vapor reflow system and subjected to a sufficiently high temperature to cause the metal content of the solder cream to liquefy and the contacts of the electronic components to be fused and to adhere to the pads on the circuit board.
Many solder creams or compositions have been developed and are commercially available. However, such prior art and commercially available solder creams have the disadvantage that during soldering many small solder balls are formed around and between each of the solder pads. Many of these solder balls create electrical short circuits between adjacent pads. Heretofore, it was necessary to scrub the circuit board and the contacts of the components with a small, stiff brush in an effort to remove the solder balls. The main problem, however, is that many solder balls form beneath the components, and it is difficult, and sometimes impossible, to scrub or remove solder balls from the areas of the circuit board immediately beneath the components. Thus, due to this problem, short circuiting exists and the rejection rate for such boards has been very high, e.g. of the order of 60%. In other instances, even though a circuit board may test satisfactorily, vibration, temperature changes and other factors may loosen other solder balls and cause failures. Under the circumstances, the removal of solder balls by conventional cleaning methods has proved unsatisfactory.
U.S. Pat. No. 3,762,965 discloses a solder composition for electronic circuitry comprising a vehicle containing an active chloride-containing compound capable of removing surface oxides of the solder metals, a thixotropic agent, rosin or a derivative thereof, and an organic solvent, and having soldering metals such as a combination of tin, lead and silver, dispersed in the vehicle. As noted in the table in the patent, the weight proportion of vehicle including rosin flux, in the compositions can range from 15 to 25%, and the range of soldering metals, from 75 to 85%.
Although the patent states that the solder composition thereof results in reduced solder ball formation, applicant has found that when employing the above noted ranges of proportions of vehicle and soldering metals, some solder balls are still formed, particularly beneath the electronic components as noted above, and which are difficult to remove.
U.S. Pat. No. 2,480,723 discloses a solder-flux mixture containing a rosin type flux, and alkaline ingredients such as ammonium hydroxide solution, and a mineral oil, having particles of solder metals such as tin, lead and silver suspended therein. According to the patent, the ratio of solder powder to the rosin is preferably around 20 to 1 or 15 to 1, at which the flux residue is not excessive upon completion of the solder bond, the patent noting that in economical practice, the ratio of solder to flux and suspending material is about 2 to 1.
U.S. Pat. No. 3,073,270 discloses a brazing paste comprising broadly from 60 to 90% of a powdery joining metal component, 10 to 40% of an emulsion vehicle, and a flux in the amount of 0 to 20% by weight of the composition.
U.S. Pat. No. 3,684,533 discloses a solder composition comprising a liquid vehicle including a rosin in a range, e.g. between 30% and 60% of the vehicle, the vehicle constituting, according to the table in the patent, between 15 and 40%, and the metals content between about 60 and 85%, by weight of the solder composition.
It is accordingly an object of the present invention to provide a solder composition or solder cream which is especially adapted for soldering components in electronic circuitry, and which minimizes formation of undesirable solder balls causing short circuits between electronic components. Another object is to provide a solder composition formed of conventional components or ingredients, yet compounded so as to reduce or eliminate solder ball formation. Yet another object is to provide a solder composition or solder cream especially designed for soldering electronic components in electronic circuitry on a substrate, which contains soldering metals dispersed in a vehicle having suitable flow and viscosity characteristics, and which contains a rosin flux, the proportions of vehicle containing rosin flux, in relation to the soldering metals, and the particular size of the solder metals, being such as to provide a composition which produces few, if any, solder balls in the spaces between electronic components, between adjacent solder pads or beneath electronic components, following the fusing or the soldering operation.